Oxalic acid is strong, and in industrial uses it is a potent cleaner and corrosive. It is naturally present in many plants, including some fruits, though not in concentrations high enough to be toxic. Concerns about oxalic acid center primarily on its tendency to combine with calcium in the body, forming salts that may contribute to kidney stones and depleting the body's store of calcium.
Function in Fruits
A study by Heather Wyman and James K. Palmer found that, as bananas ripened, quantities of oxalic acid decreased, suggesting that oxalic acid may play a role in ripening. In related research, oxalic acid was demonstrated to prevent fruits from browning, and the presence of oxalic acid seems to protect some plants from heat damage.
Fruits Containing Oxalic Acid
Determining the oxalic acid content of fruits is a difficult task. The amount of oxalic acid present may be affected by growing conditions, ripeness and preparation of the fruit. Furthermore, attempts to generate reliable data on the oxalic acid content of various food have led to highly variable results, with some tests reporting much higher or lower oxalic acid content than other tests. Using the U.S. Department of Agriculture's data as a guide, fruits that contain oxalic acid, arranged from highest to lowest oxalic acid content, include eggplant, tomato, pepper, cucumber and squash. Dr. Duke's Phytochemical and Ethnobotanical Databases adds star fruit, banana and plantain, pumpkin, mango, black currant, orange, pineapple, grape, peach and bitter melon to the list.
Multiple health concerns surround oxalic acid. First, it is a toxic chemical in large quantities, though it is impossible to consume enough fruit to reach toxic oxalic acid levels. Primary concerns focus on oxalic acid's role in worsening certain disease conditions or in leading to nutritional deprivation.
Concerns about oxalic acid originate with how the body processes it. Oxalic acid combines with calcium to form a salt called oxalate, which is processed through the body as a waste. That salt may build up in the body, however, and contribute to conditions such as kidney stones, rheumatoid arthritis and gout. The process of forming oxalate also deprives the body of calcium, raising questions about whether or not it can contribute to nutritional deficiencies.
Assessment of the risk of oxalic acid varies. Generally, people who are not prone to conditions worsened by oxalic acid, such as kidney stones, do not need to monitor their consumption of fruits containing oxalic acid because fruits tend to contain very little of the substance. The National Institutes of Health rejects any adverse nutritional effects for healthy people consuming oxalic acid in fruits and vegetables. People with medical conditions worsened by oxalic acid may need to be more careful and should consult their doctors.
Fruit Juice and Oxalic Acid
Because "juicing" results in higher concentrations of the substances contained in a particular fruit, extra care may need to be taken with respect to oxalic acid. Jesse Dallas, author of "The Juicing Book," recommends that people prone to oxalic acid-related conditions take care in juicing with fruits that contain the substance. Furthermore, care is advised when combining fruits with vegetables because many vegetables also contain oxalic acid.